1. Field of the Invention
The present invention relates to an LED drive circuit and, in particular, to an LED drive circuit for driving an LED element, for example, used as the backlight of the liquid crystal screen of a cell phone, a portable game machine, or the like.
2. Description of the Related Art
An LED element is used as a lighting element, for example, in the backlight of a traffic signal or a liquid crystal display. Also, in recent years, an LED element has been used in the backlight of the liquid crystal screen of a small-size, portable apparatus, such as a cell phone or a portable game machine. As a drive circuit for an LED element in a small-size, portable apparatus as described above, there has been disclosed an LED drive circuit that includes a booster circuit for boosting the voltage by switching the output of a battery and a constant-current circuit for driving an LED element at a constant current and drives the LED element substantially at a constant current and a constant voltage (see, for example, Japanese Unexamined Patent Application Publication No. 2002-359090).
It is known that an LED element suffers thermal damage, such as brownout, due to an increase in the temperature of internal substances included in the LED element at high temperature (for example, 30° C. or more). To avoid this, it is known that the amount of a current to be passed through must be made smaller than that at room temperatures (for example, 10° C. to 30° C.). For this reason, LED element manufacturers indicate the allowable forward current for usage. For example, FIG. 5 shows one example of the allowable forward current of an LED element. According to this example, the allowable forward current is set so that it abruptly decreases as the temperature increases, as shown by a characteristic A of FIG. 5. For this reason, in a related-art LED drive circuit, a circuit is designed so that a current having a constant value that does not exceed the allowable forward current at high temperature passes through the LED element, as shown by a characteristic B of FIG. 5.
However, driving the LED element at a current having such a value means driving the LED element at a current having a value much smaller than the allowable forward current at room temperatures. Therefore, a sufficient luminance cannot be obtained. For this reason, in order to obtain a necessary luminance, multiple LED elements may need to be used. However, in the small-size, portable apparatus field where further downsizing and layer-thickness reduction are in progress, it is required to obtain a sufficient luminance with the least possible LED elements and parts thereof.